The present invention relates to a process of fabricating UO.sub.2 pellets on the basis of an ADU (ammonium diuranate) process, and more particularly, to such an improvement as to coarsen UO.sub.2 crystals in the pellets, that is, to bring the UO.sub.2 crystals to a large grain size, thereby raising their retaining power for fission-product gas (FP gas).
In order to allow to burn UO.sub.2 pellets fuel within a nuclear reactor in a stable manner, it is desirable to decrease the FP gas release from the pellets as low as possible.
According to the results of various irradiation tests and experiments which have been conducted up to now, it is known that if the grain size of the pellets which is usually on the order of 10 to 20 .mu.m, is brought to a higher value, the retention of the FP gas is enhanced. There is some concern, however, that excessively large grain size of the pellets will result in a reduction in the mechanical strength thereof. Although an optimum grain size has not yet been ascertained, it is estimated that the target for the upper limit of the grain size is approximately 100 .mu.m.
Two methods are known as means for enlarging the grain size of UO.sub.2 pellets. In the first of the known methods, additives such as, for example, niobia (Nb.sub.2 O.sub.5), titania (TiO.sub.2), or the like, which have a grain growth-accelerating function, are added to UO.sub.2 powder. The mixture of the powder and additives are then compacted and sintered. The second known method is one in which the sintering conditions for the pellets are controlled. Specifically, an oxidizing gas, such as CO.sub.2 gas, or the like, is used as the sintering atmosphere for the pellets. A similar effect is achieved when sintering is carried out at a higher temperature and over a longer period of time than that usually employed in the sintering process (1700.degree. to 1800.degree. C. and 2 to 8 hours), thereby enhancing grain growth.
However, the aforesaid first method is not desirable, because there is a possibility that the use of the additives may alter the thermal properties of the material such as the melting point, thermal conductivity, thermal expansion, and the like. On the other hand, it is difficult to carry out the aforesaid second method because it is necessary to considerably modify the fabrication for the pellets, resulting in an increase of costs and in a sizable reduction of productivity.
In light of the above considerations, it would be desirable to have a method in which it is not necessary to alter fabrication conditions and in which no alteration of the thermal properties of the pellets results. The above demand can be carried out relatively easily by improving the performance of the UO.sub.2 powder, that is a high active powder can be used for a raw material of the large grain size pellets.
Typically, in the ADU method employed to fabricate the UO.sub.2 powder, an aqueous solution of uranyl fluoride (UO.sub.2 F.sub.2), obtained by reacting UF.sub.6 gas and water with each other, is then reacted with ammonia to form ADU precipitate. The precipitated ADU is then filtered and dried. Thereafter, the ADU is calcined and reduced, and is formed into UO.sub.2 powder. However, the UO.sub.2 powder obtained by the ADU method is relatively low in activity, and as a result, only pellets with grains on the order of 10 .mu.m can be produced.
In addition to the above-mentioned ADU method, there is also another ADU method which utilizes an aqueous solution of uranyl nitrate (UO.sub.2 (NO.sub.3).sub.2) as the raw material. In this other ADU method, UF.sub.6 gas, for example, is hydrolyzed in a nitrate solution that contains a defluorination agent, to form UO.sub.2 (NO.sub.3).sub.2 aqueous solution. This solution is then refined by means of solvent extraction. Subsequently, the aqueous solution is reacted with ammonia to form ADU. The ADU is then filtered and dried and, thereafter, is calcined and reduced to form UO.sub.2 powder. Using this method can result in a UO.sub.2 powder which is higher in activity than that obtained by the first-mentioned ADU method. It has been found, however, that even if the UO.sub.2 powder of relatively high activity is employed, only pellets having grain sizes on the order of 20 .mu.m are obtained in practice. This fact suggests that in order to increase grain size, it will be necessary not only to raise the activity of the UO.sub.2 powder, but also to raise cohesiveness, and the like, of the powder. Thus, it has been difficult for each of the conventional UO.sub.2 powder fabrication processes to produce pellets having grain equal to or larger than 20 .mu.m.